re-entry time calculaion - example from Hartman

[CoffeeBreak]

Hello everyone;

Can anyone post a solution to this example from Hartman's ventilation book:
Blasting in a raise liberates 200 ft^3 of fumes. Raise dimensions are 4 ft x 6 ft in cross section and raise is 40 ft above the level. Auxilliary ventilation supplies 800 cfm of fresh air.
How long will it take to dilute the fumes to 100 ppm so the miners can return to work?

The result is 9.2 min but I can't get that number...

Any help really appreciated.

Thanks !!!